Nucleation regions and coercivity determination of sintered Nd-Fe-B magnets

This paper describes the use of magnetic force microscopy MFM to observe magnetization reversal and nucleation in sintered permanent magnet materials. The investigated magnet is sintered Nd-Fe-B magnet. The domain structure is observed in both perpendicular and parallel surfaces to the alignment axis (easy axis) in a thermally demagnetized state. Maze-like and stripe-like magnetic domains are observed in perpendicular and parallel surfaces respectively. The nucleation regions are successfully observed at domain walls and presented in 3D with clear magnetic contrast images. The sample shows narrow magnetic domains with negative nucleation regions. The volume of nucleation site are determined and validated theoretically by developing an approach involving coercivity versus nucleus volume. It is shown that nucleation volume is proportional to coercive field. These insights can be useful for realizing Dy free Nd-Fe-B magnet with high coercivity.